A lot depends on the year of the boat, DS II built before 1980 the board is held i nwith a bolt through the CB trunk, 1980 and later the CB pivots on a pin that is accessed from under the boat. In both cases it wil lbe best to haul the boat out, can you back your trailer down a beach or the shore? If so, yo ucan most easily d owork on the CB by bringing the trailer (don't need a vehicle) to the waters edge and winch the boat on until the forward end of the CB trunk is just aft of the aft most roller on the trailer (you could also do this on a grassy area). This supports the boat pretty good yet allows great access to the CB.
OK, let me backtrack a bit....... (I got ahead of myself!) First, yo usay the CB is stuck in the up position. When you dove under the boat, could you free the CB? I had to use gentle persuasion with a small pry bar the first couple times this happened. If you were able to get the CB to come down by wiggling/prying from below..... and if the lines/cables weren't jamming it, then I suspect that you have a CB like mine.... top forward corner of the CB is squared off..... inside of CB trunk is rounded, CB jams. I had to remove the CB and sand/grind the edges a bit to round them over.... CB is now less likely to jam. Next, it is possible that you are getting a vacuum forming in the trunk due to air not being able to get in to break it. The line that is attached to the CB to lower it (that is pull it down) is a 5/16" 3-strand line and that is a slightly tight fit through the "donut" in the CB trunk, try carefully prying the donut up and out (temporarily) that may let in a bit of air and allow the CB to drop easier. You say the line is "frayed", when you remove the CB to sand the upper corner, replace that down line with a length of 1/4" double-braid polyester line (about 30" long) the slightly smaller diameter will allow more air in around the opening. Finally, there may be some kind of dirt / debris / growth that has jammed the CB.
Now ,getting back to the haulout to fix. You could roll the boat over on her side if you have a wall to lean her against ,or have someone sit on the top of the mast (not sure how good that idea is for the mast and rigging?). I prefer working with the boat upright. If your boat is a pre-1980 model (your description does tell me that she is theoretically at least 1974 or newer) the CB is removed by removing the bolt that holds the CB in place. The bolt is located under the cockpit sole and is accessed through the 2 inspection/bailing ports in hte forward end of the cockpit. you need two 3/4" wrenches (adjustable is fine for one, but a socket/ratchet will be best for the other) The bolt is a bit long (at least on my boat!) but should come out OK. Before removing the bolt, put some tension on hte line that pulls the CB down, it wil lstop the CB from dropping out as soon as you remove hte bolt. There is a rubber washer and a stainless-steel flatwasher on both sides of the CB trunk (don't lose them!). On a 1980 or newer boat, the CB pivots o na plastic (nylon?) pin that is accessed from under the boat. You will see 2 stainless-steel plates, one on each side of the CB slot, remove the screws holding them (3 -each, I think) then you will see 2 fiberglass wedges up in the CB trunk, they hold the CB pin in place. you may need to slip a putty knife or other thin blade in there to pry them from the sides of the recesses that they fit into. Once they are out, the CB should slide out.
Now let down on that line to allow the Cb to slip out, on 1979 or older boats the CB actually pivots on a nylon bushing, as the CB comes out this bushing may slip out, don't lose it! On all years (1974 and later) the line to pull CB down will need to be untied from the block (pulley) that is tied to it. Let the line slip through the Donut and out (not a bad idea to tape a string to it first... to pull line back in when reinstalling CB). O ntop of the CB wil lbe a Tang attached to the CB by 3 screws, remove hte 3 screws and the CB is free. Putting the CB back in is just the reverse procedure (if only it were THAT simple!) Let me know if you want a better description!
I have removed/reinstalled my CB many times, once partially removing it to replace the up cable while the boat was half on the trailer at the waters edge! That reminds me! Check that cable.... now is the time to replace it if needed, while you have it all apart anyway!
